Someone asked me this question recently and I had an immediate answer: commas. When I went back to school to finish my degree, comma splices were my nemesis! It would nearly kill me when I would lose a 1/2 or full point of my grade because of those commas!! There are so many rules and ways of using commas (and not using them), that I periodically go back and refresh my comma skills.
Now it is your turn: What is YOUR biggest grammar issue, difficulty or pet peeve?
You can let me know in the comments or contact me privately. I will do my best to explain those issues in future difficulties. 🙂
Tweetable: What is Your Biggest Grammar Issue?